BCECF, AM is a cell-permeant, dual-excitation ratiometric pH indicator with a pK_a of ~6.98 and is ideal for measuring changes in the cytosolic pH of most cells. Intracellular pH measurements with BCECF are made by determining the pH-dependent ratio of emission intensity (detected at 535 nm) when the dye is excited at ~490 nm versus the emission intensity when excited at its isobestic point of ~440 nm.